1. Field of the Invention
This invention relates to a vacuum fluorescent print head for printing paper having luminous elements with phosphorous objects which emits light beams to the printing paper based on image data, the luminous elements being arranged zigzag and in a plurality of columns extending in a main scanning direction.
2. Description of the Related Art
A print head for use on a fluorescent printer for forming color images on a photosensitive medium is disclosed in U.S. Pat. No. 5,592,205 (corresponding to Japanese Patent Laying-Open Publication H5-92622), for example. This print head has filamentary electrodes acting as cathodes for releasing thermions, control electrodes, and a plurality of strip-like anode electrodes covered by phosphorous objects of a predetermined size arranged at predetermined intervals, all sealed in a vacuum case. Thermion impingement upon the phosphorous objects, i.e. light emission from the phosphorous objects, is controlled by applying a voltage to the strip-like anode electrodes and applying control signals based on image data to the control electrodes. Each phosphorous object corresponds to one pixel of an image, i.e. one dot. The phosphorous objects must be arranged close to one another to obtain high resolution. However, it is essential that the phosphorous objects are spaced from one another. It is thus necessary to arrange the phosphorous objects zigzag and in a plurality of columns extending in the main scanning direction, such that the intervals between the phosphorous objects in each column are covered by the phosphorous objects in another column.
In the above print head having the phosphorous objects arranged zigzag and in a plurality of columns, the phosphorous objects in one column partially overlap the phosphorous objects in another column in order to avoid gaps occurring, in a sub-scanning direction at right angles to the main scanning direction, between light beam dots formed on the photosensitive medium by the phosphorous objects. Such a print head is effective as a writing head for an electronic copier, for example. However, when used in a digital exposing apparatus for processing photographic printing paper, such a print head causes double exposure where the light beams overlap one another on the printing paper. The overlapping positions have increased density, resulting in stripes due to density variations from dot to dot on the printing paper.